The present invention relates generally to floating surface aerators for mixing oxygen with treated wastewater in a wastewater treatment facility and, in particular, to such an aerator that is also an anoxic mixing device.
In a sequencing batch reactor (SBR) for treating wastewater with activated sludge, wastewater, during a fill period of an SBR treatment cycle, is introduced into the reactor having a layer of sludge therein. This "fill" period is primarily anoxic or without aeration and a mixer may be utilized to stir and intermix the sludge and incoming liquid. Aeration of the mixture is then initiated. Aeration is necessary because the microorganisms in the sludge require oxygen to sustain their metabolic function, allowing them to grow and multiply so as to rapidly consume organic material in the waste that the bacteria feed upon. When aeration is discontinued, the mixed liquor is allowed to settle under quiescent conditions so as to separate sludge from clarified fluid. A portion of the clarified liquid with contaminants substantially removed is then decanted.
It is important that during the anoxic fill and mixing period, the liquid is not aerated so that there is a wide swing in the food-to-microorganism (F/M) ratio and the oxygen uptake rate of the wastewater and sludge between this fill and/or mixing period and the aeration period as the bacteria require oxygen in order to consume the waste. During the anoxic fill and mixing period, the F/M ratio is high, setting the stage for a "feast" when aeration of the basin begins. During the "feast" period of aeration, the dissolved oxygen level stays near zero because the oxygen demand exceeds the aeration system's maximum capacity. This is beneficial because the growth of unwanted, slow-growing filaments is inhibited and the growth of beneficial, floc-forming, zoogleal organisms is encouraged.
Typically, an SBR contains an anoxic mixing device and a separate aeration device. As indicated in the previous paragraph, this separation is important so that the mixing device does not promote aeration. It would be economically advantageous, however, to have one device that can both selectively anoxically mix and later aerate the liquid in a wastewater reactor. It is noted that it is also possible to use alternating aeration and mixing in certain operational modes to encourage removal of nitrogen or for other operational considerations which may be accomplished by such a single device.